Disclosed herein is an improved method of plastic injection molding of plastic parts with a critical external dimensional accuracy, including plastic gears, wherein a plastic molding material is first injected into a mold, and wherein then gas is injected under pressure into the mold to reduce shrinkage of the plastic material away from the mold surface, forming a gas bubble in said plastic molding material in said mold, the improvement comprising the further step of injecting a plastic molding material under pressure into said gas bubble in said plastic molding material in said mold before removing said plastic molding material from said mold.
By way of background, in known molding methods, including plastic molding by screw injection, the screw injector can continue to supply additional plastic under pressure into the mold cavity to keep it filled as the plastic material cools and solidifies. However, once a certain thickness of the outer surfaces of the material being molded solidifies (the material contacting the mold surface normally solidifies first) the additionally injected plastic is often insufficient to keep the material from shrinking away slightly from the mold cavity surface during further cooling. Thus, the finished molded part does not fully conform to the mold dimensions.
It is known that this problem can be overcome by injecting a gas, with sufficient pressure (rather than further mold material) into the interior or central portion of the mold as the material cools. There is documented research on such plastic injection molding assisted by pressurized gas injection for reducing dimensional variations in molded parts. It can improve maintaining conformity of the molded part exterior surfaces to the interior surfaces of the mold by reducing the shrinkage the exterior of the molded part away from the mold surface during cooling. For practical reasons in the technology, a higher pressure can be obtained with injected gas than with the injected plastic. Some examples of U.S. patents on plastic injection molding which is gas assisted (which patents also teach examples of control systems for regulating the timing and flow of the gas with respect to the plastic) are: U.S. pat. No. 5,707,659xe2x80x94xe2x80x9cFlow front control system for a gas assisted plastic injection molding apparatusxe2x80x9d;U.S. Pat. No. 5,639,405xe2x80x94xe2x80x9cMethod for providing selective control of a gas assisted plastic injection apparatusxe2x80x9d; and U.S. Pat. No. 5,151,278xe2x80x94xe2x80x9cPlastic injection molding nozzle with gas assistxe2x80x9d. In view of these and other references cited therein there is no need to re-describe herein in any detail to those skilled in the art suitable such gas assisted plastic injection molding for increasing of the interior pressure to reduce shrinkage.
Gas assist is typically a delayed injection process through its own set of drafts into the mold. Suitable gas injection plastic molding may be done as follows. (This is merely an example.) The mold material, in liquid form, is injected into the mold. Upon cooling the material tends to retract from the mold surface toward either the center of the part, or back into the draft columns. Thus, immediately after the material injection ends, i.e., before any shrinkage can occur, a gas is injected into the mold at a higher pressure than the material injection pressure. This gas forces its way in along a path of least resistance forming a gas bubble inside the material. Thus, the bubble tends to stay in the approximate center of the part, away from rigid bodies such as the mold walls. However, the bubble is not uniform or concentric. Variations in part thickness may also affect the spreading of the gas bubble. The gas is held under pressure in the mold until the plastic material has cooled. The result is that the gas will have exerted enough pressure from the inside of the part that so the shrinkage on the outside of the part will be nearly non-existent. This means that a perfect mold can provide a near perfectly conforming gear or other dimensionally critical molded part.
With such gas assisted plastic injection molding the compressed density of the gas, hence the much higher pressure it dispenses, does increase the accuracy of the molding process. However, there is a disadvantage in doing so. Namely, the relative strength of the part is reduced, due to a hollow central area formed by a gas bubble.
This problem is particular acute for molded parts with critical dimensions, especially, plastic gears. Molding the outside dimensions of a plastic gear more accurately can significantly increase the motion and transmission quality of the gears through mesh (as they rotate). However, prior gas assisted molding has heretofore not been useful for plastic gear production. The gas bubble inside the molded gear significantly reduces the strength and life of the gear. Also, since the bubble varies in position and size, it causes an undesirable variability in the rotational moment of inertia of the gear, similar to gear runnout.
Thus, a specific feature of the specific embodiment disclosed herein is to provide in a method of plastic injection molding of plastic parts with a critical external dimensional accuracy, including plastic gears, wherein a plastic molding material is first injected into a mold, and wherein then gas is injected under pressure into the mold to reduce shrinkage of the plastic material away from the mold surface, forming a gas bubble in said plastic molding material in said mold, the improvement comprising the further step of injecting a plastic molding material under pressure into said gas bubble in said plastic molding material in said mold before removing said plastic molding material from said mold.
Further specific features disclosed herein, individually or in combination, include those wherein said further step of injecting a plastic molding material under pressure into said gas bubble utilizes the same said plastic molding material first injected into said mold; and/or wherein said further step of injecting a plastic molding material under pressure into said gas bubble in said plastic molding material in said mold before removing said plastic molding material from said mold is performed after said first injected plastic molding material has substantially hardened in said mold.